1. Technical Field
The present invention relates to a resonant circuit for extracting a signal at a hit frequency from a data flow, e.g., P.C.M., and for assuring a good performance level in terms of frequency selection and stability during changes in environmental conditions particularly during temperature changes. The resonant circuit according to the present invention comprises a short-circuited line section fixed on a quartz substrate; in the relevant extraction system, it is preceded by a data flow input circuit and followed by an output circuit amplifying the signal extracted by said resonator.
2. The Prior Art
It is well-known that a transmission line section closed in a short-circuit has resonant characteristics and therefore presents a band-pass circuit with respect to the signal component present at its input, at the frequency fo, which corresponds a wave length ".lambda." equal to four times the length "l" of said line section. In other words the oscillation filtered by the resonator with a line length of "l" has the frequency: fo=Vp/.lambda.=Vp/(41), where Vp is the propagation speed of an electromagnetic wave transmitted by the line and depends substantially on the material used as dielectric substrate. Conversely, when it is desired that a line section extract a component having frequency fo, its length "l" must be equal to: 1=Vp/(4fo). In practice these properties can be utilized when they do not involve too high "l" values, that is, when fo is very high; in fact the use of resonant lines has been, until now, confined within the microwave field (that is, within the very high frequencies corresponding to very short wave lengths). However, there are data transmission systems of the PCM type which operate in frequency ranges corresponding to wavelengths that are well below the "microwave" region and these systems are becoming more common. Therefore, clock signal extraction is usually carried out by means of LC resonant circuits having lumped parameter components and, if necessary, with distributed inductances L (bobbins in spiral form). These conventional resonators present several drawbacks, among which can be mentioned the inconvenience caused by low selectivity due to limited Q-factors of components, and by signal irradiations in air, mainly when relatively high frequencies (but still well under the microwave frequencies) are employed, such as the frequency involved in a line system at 565 Mbit/s.